In modern low voltage electron microscopes, especially the chromatic unsharpness, and especially the axial chromatic aberration of the first order, limits the achievable resolution to approximately 3 nm for 1 keV electron energy. The diameter d of the disc of the chromatic aberration in the Gaussian image plane of an objective can be described as a product of the chromatic aberration constant CF, the aperture xcex1 of the electron beam in the Gaussian image plane and the relative energy width xcex4E/E of the electron beam.
In the utilized rotation-symmetrical electron lenses, the chromatic aberration constant is in principle unequal to zero because of the Scherzer theorem and the electrons are not monochromatic because of the emission process and the Boersch effect, that is, the anomalous broadening of the energy distribution because of stochastic Coulomb interaction so that the relative energy bandwidth is always unequal to zero. The chromatic aberration constant is approximately 1.5 mm for electrons having an energy of 1keV and for a working distance of 2 mm in modern low voltage scanning electron microscopes having an optimized objective lens and a zirconium oxide-tungsten-Schottky emitter as an electron source. The energy width xcex4E amounts to approximately 0.5 to 1 eV in dependence upon the beam current.
A further minimization of the chromatic aberration constant is hardly possible. For this reason, it is already known, in order to further increase the resolution, to utilize chromatic aberration correctors which comprise either quadrupole lens systems or electrostatic mirrors. Chromatic aberration correctors are as a rule very complex with respect to their configuration and, as an alternative thereto, the energy width of the electron beam, which is processed subsequently by the downstream electron-optical imaging system, can be reduced with the aid of a monochromator.
Rotationally symmetrical electrostatic filter lenses are known as monochromators having a relatively simple configuration. In these filter lenses, those particles are filtered out whose kinetic energy lies below the energy defined by the electrostatic potential difference. These particles are filtered out by an electrostatic field directed in opposition to the propagation direction of the particles. Such electrostatic filter lenses have, however, the disadvantage that the beam brightness of the electron beam can be greatly deteriorated.
Furthermore, Wien filters are known as monochromators for charged particles wherein an electrostatic dipole field and a magnetic dipole field are superposed perpendicularly to each other. From the article in the xe2x80x9cJournal of Physics E: Scientific Instrumentsxe2x80x9d, Volume 3 (1970), starting at page 121, it is already known to seat two Wien filters arranged one behind the other. The transmitted specimen is mounted between the two Wien filters. The second Wien filter functions as an energy analyzer to generate the electron energy loss spectrum of the specimen. The Wien filters are arranged between two quadrupole lenses in order to compensate the focusing action of the Wien filters in one direction, namely, in the direction of the electrostatic field.
In a more recent conference contribution by Tanaka et al (published in the Institute of Physics Conferences Series No. 165, Symposium 6, pages 217 and 218), a further monochromator is suggested which comprises two Wien filters arranged one behind the other. The energy selection takes place in the center between the two Wien filters. The second Wien filter functions to compensate the astigmatic imaging generated by the first Wien filter.
In a further conference contribution by Terauchi and Tanaka (published in the same volume, pages 211 and 212), a configuration of a Wien filter is described wherein the surfaces of the electrostatic electrodes as well as the surfaces of the pole shoes of the magnetic dipole are configured as cylindrical surfaces so that maintaining the Wien condition is ensured also in the fringing field regions and to produce stigmatic imaging.
It is an object of the invention to provide a monochromator having a simple configuration which is simple to adjust and has no disadvantageous influence on the beam brightness of the filtered particle beam.
The monochromator of the invention is for charged particles having a direction of propagation. The monochromator includes: a plurality of Wien filters defining an optical axis and being arranged serially one behind the other in the direction of propagation; and, a first portion of the Wien filters being mounted azimuthally rotated about the optical axis by 90xc2x0 relative to a second portion of the Wien filters.
According to a first aspect of the invention, the monochromator preferably exhibits four Wien filters arranged in series one behind the other of which one portion is rotated azimuthally by 90xc2x0 about the optical axis relative to the other Wien filters. With this mutually opposed rotated arrangement of the Wien filters, it is achieved that a portion of the Wien filters, preferably the two outer Wien filters, generate an astigmatic imaging in one direction and the other Wien filters (preferably, the center Wien filters) generate an astigmatic imaging in the direction perpendicular thereto so that, in total, a stigmatic imaging is achieved. According to a first aspect of the invention, the monochromator itself can image stigmatically. For this reason, no further elements, especially no additional quadrupoles, are needed for a stigmatic imaging.
The fields in the outer Wien filters should be aligned parallel to each other and the fields in the center Wien filters should be directed antiparallel to each other. Additionally, the refractive forces of the individual Wien filters should be so selected that one of the two axial fundamental paths of the monochromator is symmetrical to the center plane of the monochromator and the axial fundamental path in the intersect plane perpendicular thereto is antisymmetric to the center plane of the monochromator. In this way, it is achieved that the imaging, which is generated by the monochromator, in total is also dispersion-free. Energy fluctuations then lead to no time-dependent movement of the image.
An astigmatic intermediate image is generated in the center plane of the monochromator whereat the energy filtering can take place by a slit diaphragm.
The refractive power of the outer Wien filters should be selected greater than the refractive power of the inner Wien filters in order to generate stigmatic imaging.
According to a second aspect of the invention, with one or several Wien filters arranged one behind the other in the propagation direction, the pole shoes of the magnetic dipole(s) of the Wien filter(s) and the electrodes of the electric dipole(s) of the Wien filter(s) are configured symmetrically to each other with respect to an azimuth rotation by 90xc2x0 about the common optical axis. For this purpose, the electrodes of the electric dipoles should be made of the same soft magnetic material as the pole shoes of the magnetic dipoles. Because of the symmetrical configuration of the magnetic dipoles and the electric dipoles to each other, it is achieved that the magnetic and electrostatic potential distribution and also the magnetic and electrostatic field distribution are the same in each longitudinal direction as well as in each transverse direction.
The symmetry of the magnetic and electrostatic dipole fields also in the fringing field regions leads, on the one hand, to the situation that the imaging errors are avoided. Simultaneously, the symmetrical configuration of the electrostatic portion and of the magnetic portion of the individual Wien filters affords manufacturing advantages in the serial arrangement of several Wien filters rotated relative to each other. The electrodes and pole shoe parts of all Wien filters to be combined with each other can be manufactured from one material block which, in a first processing step, is provided with a central longitudinal bore for the passage of the electron path and further longitudinal bores for the later reassembling.
Only in a second step, the electrodes and the pole shoes of all Wien filters, which are to be combined with each other, are machined out and, only in a third step, the block of material is taken apart perpendicularly to the bores generated in a first step. In a subsequent step, the coils and the electric contacts are applied and the insulation is applied between the individual pole shoes. With the aid of the assembly bores generated in the first step, it is ensured in a later reassembly that the arrangement is mutually symmetrical.
For a correct adjustment of the Wien condition also in the fringing field regions of the Wien filters, the surfaces of the pole shoes, which are directed to each other, and the electrodes should be configured as concentric spherical surface sections or coaxial cylinder surface sections. The concentric spherical surface sections or the coaxial cylinder surface sections of the surfaces of the pole shoes and electrodes directed to each other can, however, also be approximated by a plurality of polygonally arranged surfaces which are planar. Furthermore, the individual pole shoes of each Wien filter should be electrically insulated with respect to each other and diaphragms made of a material of high magnetic permeability should be arranged between the individual Wien filters in order to prevent cross talking of the magnetic fields between the Wien filters.